West Yorkshire Police were called just after 2.30am to a report of a collision on the A642 Wakefield Road in Lepton.
A spokesman said a man had been hit by a lorry on the A642 in Lepton, heading towards Huddersfield.
"The male suffered life-threatening injuries," he said.
The road remains closed while investigation work is carried out at the scene.
The spokesman said: "This closure is expected to remain in place until later this morning.
"As a result drivers are urged to find other routes while this necessary investigation work is carried out."